Ask The Perf Guy: What’s The Story With Hyperthreading and Virtualization?

There’s been a fair amount of confusion amongst customers and partners lately about the right way to think about hyperthreading when virtualizing Exchange. Hopefully I can clear up that confusion very quickly.

We’ve had relatively strong guidance in recent versions of Exchange that hyperthreading should be disabled. This guidance is specific to physical server deployments, not virtualized deployments. The reasoning for strongly recommending that hyperthreading be disabled on physical deployments can really be summarized in 2 different points:

  • The increase in logical processor count at the OS level due to enabling hyperthreading results in increased memory consumption (due to various algorithms that allocate memory heaps based on core count), and in some cases also results in increased CPU consumption or other scalability issues due to high thread counts and lock contention.
  • The increased CPU throughput associated with hyperthreading is non-deterministic and difficult to measure, leading to capacity planning challenges.

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